Metal coating process



3,015,579 METAL COATING PRQCESS Rfaurice R. Commanday, Los Angeles, andJames R.

Darnell, Reseda, Califi, assignors, by mesne assignments, to ChromizingCorporation, a corporation of Delaware No Drawing. Filed June 15, 1959,Ser. No. 820,122 8 Claims. (Cl. 117-107) This invention has to dogenerally with improved surface treatments of the refractory metalscolumbium and tantalum to prevent their destructive oxidation whenexposed to air at high temperatures, and to reduce their tendency towardembrittlement through the absorption of gases.

It is well known in the art that the refractory metals, including alsotungsten and molybdenum, can be rendered resistant to high temperatureoxidation, by so-called siliconizing of the metal surfaces by eithervapor phase (vapor plating) or solid pack techniques, according to whicha silicon halide is caused to react with the metal surface to depositsilicon and form a coating complex that may be composed of silicon,silicides or silicon alloys of the base metal.

We have found that for our purposes the pack method is advantageous overthe vapor plating methods in that lower temperatures are employed,equipment is much simpler and more economical, continuou attendance ofan operator is not required, and the size of the part that may betreated is not limited to the extent required in the vapor plating typeof treatment. We have also found that in the pack siliconizing process,much greater dependency is placed upon reaction between the base metaland the active gases (silicon halide), and that the chemical propertiesof the base metal with respect to these gaseous components significantlyaftect the result. In vapor plating, the silicon halide gas is believedto be reduced by hydrogen and deposited, as in the nature ofelectroplating, upon the base metal being treated, whereas in thepacktype process, a reduction reaction or interchange reaction occurs.As a consequence of this difference, we have found it dttlicult toachieve the highest success desired in the surface treating and coatingof columbium and tantalum with simple pack siliconizing treatments,whereas similar techniques have proven quite satisfactory as applied tomolybdenum and tungsten. Our general object in the present invention isto modify the surface of columbium or tantalum parts, so that they willbe receptive to the pack siliconizing processes at practically desirabletemperatures. In referring to the metals columbium and tantalum, weintend to also include their alloys, by which term i meant alloycompositions containing preponderantly the base metal together with anyof other various metals depending upon the nature and properties of thealloy desired. For example such alloys may be composed of eithercolumbium or tantalum together with lesser percentages of the othermetals or such non-refractory metals as iron, nickel, chromium,zirconium and others.

In accordance with the invention we first difiuse molybdenum orferro-molybdenum (which may contain about 55 to 65% molybdenum and about35 to 45% iron), into the surface of the columbium or tantalum by meansof a pack difiusion application preceding the siliconizing treatment.This preliminary diffusion treatment produces what may be regarded as aprimer coating of alloy rich molybdenum or molybdenum and iron, havingpeculiarly advantageous chemical properties for reaction with siliconhalide gases in a subsequent pack siliconizing process to finally form asatisfactory protective coating. The approach taken may appearunorthodox in that we employ the most reactive of all the refractorymetals (molybdenum) insofar as its tendency for susceptibility to highnited States Patent temperature oxidation is concerned, to provide abasis for forming an oxidation-resistant coating on another refractorymetal somewhat less prone to oxidation. However, the more active anelement is in its pure state, generally the more stable are itscompounds. Thus by surface alloying the less active element, e.g.columbium, with the more active element molybdenum, we are able to formmore stable intermetallic compounds (molybdenum silicides) that exhibitsuperior protective properties, and do so by the simple solid packprocess.

We have also found that the presence of iron in the primer coating tendsto prevent embrittlement of the base metal, apparently due to absorptionof gases during treatment or subsequent heating. This is particularlyimportant in the instance of columbium, tantalum and their alloys, asroom temperature ductility is a major advantage in these metals.

In more specific reference to the methods and materials employed incarrying out the invention, the refractory base metal piece is firstferromolybdenumized by heating at a temperature in the range of about1600 F. to 2800 F. for from 3 to 20 hours in a powder mixture composedof about 30 to 50 parts by weight of farm-molybdenum, 70 to 50 parts byweight of inert diluent, and about 0.02 to 5 parts of an inorganichalide, all solids being powdery and under about 60 mesh particle size.The inert diluent may be any of various materials known to be usable inthe solid pack type of treatment, such as tabular alumina, silica,bentonite, bauxite, kaolin, crushed fire clay, chromite and the like.For our purposes, tabular alumina may be regarded as preferred. Thehalogen-containing component may be any of the different inorganichalides such as the halides of iron, copper, or ammonia, the anionichalides of course being chlorine, bromine, fluorine and iodine. Weprefer the use of the ammonium halides, and specifically ammoniumbromide.

The pack method involves placement of the refractory metal piece orpieces to be treated, in surface contact with the powder mixture in animpermeable box or retort of an appropriate heat-resistant metalprovided with a fusible rim seal that melts during the heating cycle toallow excess gase to vent, and which solidifies upon cooling to preventair from entering the box. The physical characteristics of this type ofpack have long been known in the art.

Following ferromolybdenumizing of the base metal as described, the metalthus initially surface treated is removed from the pack and siliconizedby a similar pack technique according to which the work metal is placedwithin the same kind of fusible rim box in contact with a powder mixtureof from about 20 to 50 parts of silicon, 70 to 45 parts of inert fillersuch as tabular alumina, and 0.02 to 5 parts of an inorganic halide. Asheretofore indicated, siliconizing of the previously treated metalsurface results in the formation of a final coating on the base metalrendering the latter satisfactorily resistant to oxidation at hightemperatures.

As specific examples of refractory metals treated in accordance with theinvention, we prepared three samples, the first being composed of 0.5weight percent zirconium, and the balance columbium; a second containing0.5 percent Zirconium, 39.5 percent tantalum and the balance columbium,and a third sample analyzing 16 percent molybdenum, 5.5 percent iron,3.5 percent tantalum and the balance columbium. Specimens of each ofthese alloys were ferromolybdenumized by heating at 1900 F. for sixhours in a powder mixture pack composed of 35 percent ferro-molybdenum(58.8% molybdenum and 41.2% iron), 65 percent tabular alumina and 0.3percent ammonium bifluoride. Subsequently, the ferromolybdenumizedspecimens were siliconized by heating at 1850 F. for eight hours in apowder pack mixture composed of 35 percent elemental silicon, about 65percent tabular alumina and 0.3 percent ammonium bifluoride. Thespecimens were subjected to static oxidation in air at 2400" F. with noobservable breakdown of the coating or oxidation of the base metal, forperiods up to one and one-half hours.

We claim:

1. The process of treating a refractory metal of the group consisting ofcolurnbium, tantalum and alloys thereof to render the metal surfaceresistant to oxidation at high temperatures, that includes, heating themetal to a temperature between about 1600 F. and 2800 F. in anon-oxidizing atmosphere and in surface contact with a powder mixture ofabout 30 to 50 weight parts of a substance of the class consisting ofmolybdenum and ferro molybdenum, about 70 to 50 parts of inert fillerand 0.02 to parts of an inorganic halide and thereby alloying molybdenuminto the metal surface; and subsequently heating the metal to atemperature between about 1600 F. and 2800 F. in a non-oxidizingatmosphere with said surface in contact with a powder mixture of about20 to 50 parts of silicon, 70 to 45 parts of inert filler'and 0.02 to 5parts of an inorganic halide to thereby form a molybdenum silicideprotective surface coating.

2. The process of claim 1, in which said metal is columbium.

3. The process of claim 1, in which said metal is tantalum.

4. The process of claim 1, in which said ferromolybdenum contains about55% to 65% molybdenum and about to iron.

5. The process of claim 1, in which said inert filler is alumina.

6. The process of claim 1, in which said halide is ammonium bromide.

7. The process of claim 1, in which said metal is columbium, said inertfiller is alumina and said halide is ammonium bromide.

8. The product made according to the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS2,771,666 Campbell et a1. Nov. 27, 1956 2,825,658 Samuel Mar. 4, 19582,839,292 Bellamy June 17, 1958 2,876,139 Flowers Mar. 3, 1959 FOREIGNPATENTS 493,758 Canada June 16, 1953

1. THE PROCESS OF TREATING A REFRACTORY METAL OF THE GROUP CONSISTING OFCOLUMNIUM, TATALUM AND ALLOYS THEREOF TO RENDER THE METAL SURFACERESISTANT TO OXIDATION AT HIGH TEMPERATURES, THAT INCLUDES, HEATING THEMETAL TO A TEMPERSTURE BETWEEN ABOUT 1600* F. AND 2800* F. IN ANON-OXIDIZING ATMOSPHERE AND IN SURFACE CONTACT WITH A POWDER MIXTURE OFABOUT 30 TO 50 WEIGHT PARTS OF A SUBSTANCE OF THE CLASS CONSISTING OFMOLYDDENUM AND FERRO MOLYBDENUM, ABOUT 70 TO 50 PARTS OF INERT FILLERAND 0.02 TO 5 PARTS OF AN INORGANIC HALIDE AND THEREBY ALLOYINGMOLYBDENUM INTO THE METAL SURFACE; AND SUBSEQUENTLY HEATING THE METAL TOA TEMPERATURE BETWEEN ABOUT 1600* F.AND 2800* F. IN A NON-OXIDIZINGATMOSPHERE WITH SAID SURFACE IN CONTACT WITH A POWDER MIXTURE OF ABOUT20 TO 5 PARTS OF AN INORGANIC HALIDE TO THEREBY FORM A MOLYBDENUMSILICIDE PROTECTIVE SURFACE COATING.